1. Field of the Invention
The field of invention comprises devices for damping vibrations of structures caused by seismic shock or the like.
2. Description of Related Art
A variety of active and passive seismic response control systems are known, including variable stiffness devices, to provide for the safety of structures. For instance, a variable stiffness earthquake-resisting mechanism may be integrated in a column-and-beam type frame structure in the form of an adjustable brace in which the rigidity of the variable stiffness mechanism, or the means of connection between the frame and the variable stiffness mechanism, functions to analyze seismic vibrational forces and to provide damping to offset these forces.
Prior art active seismic response control systems attempt to deal with seismic vibrations by actively shifting the natural frequency of the structure against the predominant period of a seismic vibration. However, seismic motion is an irregular vibration which does not have a clear predominant period, and in some instances, the predominant period is plural. Furthermore, in the case of prior art active seismic response control systems, various sensors as well as a controlling computer are used. To safeguard against the possibility of unforeseen events, a variety of safety maintenance mechanisms are necessary, the control of which becomes complicated. These safety mechanisms are not only costly, but require valuable start-up time to become effective. During this start-up period, the structure is either unprotected or not fully protected.
For instance, Kobori et al. U.S. Pat. No. 4,890,430 discloses an active damper which is computer controlled to vary the natural resonance of an entire building by actively varying the rigidity of selected structural members. Kobori. et al. U.S. Pat. No. 5,022,201 is an active seismic damper comprising a mass damper mounted on the top of a building. The damper is actively vibrated by an actuator connecting the mass to the building. Ishii et al. U.S. Pat. No. 5,025,599 discloses a combination active and passive damping device wherein a mass damper is rendered actively vibratable by a hydraulic actuator. In the event of a power failure, the device is converted to a passive damper wherein the mass is passively vibratable by coiled springs between the mass and the building which are excited solely by the energy of seismic vibration.